Last fall, Maine voters passed an experiment in voting that no state has ever before tried: ranked-choice voting. It’s an experiment some say could change the national calculus against third parties, as I’ll explain below. But the state’s Republican-led Senate asked Maine’s Supreme Court to rule on the system — and the court recently issued an advisory ruling that ranked-choice violates the state constitution.

So why would anyone be interested in ranked-choice voting — and why are Maine’s Republicans fighting it?